A system configuration in which a plurality of distributed units equipped with some of base station functions are extended in a wireless communication system, particularly a mobile communication system, has been studied. A base station obtained by extending a distributed unit has a function of controlling a plurality of distributed units as a central unit. In a communication scheme of performing communication between the central unit and the distributed units, two system configurations such as full centralization and partial centralization, which differ in a function distribution between the central unit and the distributed unit, are being studied.
FIG. 3 is a block diagram illustrating a system configuration of full centralization. In the system configuration of full centralization, as illustrated in FIG. 3, the central unit 301 is equipped with a physical layer function unit 11 from which an antenna unit is excluded and a functional unit 12 of a data link layer or a higher layer. Also, in the system configuration, n (n is a natural number) distributed units 302-1 to 302-n equipped with only an antenna unit 21 are extended.
FIG. 4 is a block diagram illustrating a system configuration of partial centralization. In the system configuration of partial centralization, the functional unit 12 of the data link layer or the higher layer and some functions 13 of physical layer functions are installed in the central unit 401 as illustrated in FIG. 4. Also, in the system configuration, distributed units 402-1 to 402-n equipped with some functions 22 of the physical layer functions are extended (for example, see Non-Patent Document 1).
Also, in a communication system, error correction code technology for correcting an error in a transmission path by assigning redundant bits to a signal bit sequence to be transmitted is known (for example, see Non-Patent Document 2). In this error correction code technology, a signal bit sequence to which a redundant bit is assigned by a transmission device is modulated into any form, transmitted through a transmission path, and received by a reception device. At this time, when the received modulated signal is demodulated, a process of determining whether a signal bit is 0 or 1 on the basis of a certain threshold value and outputting a determination result as a received bit sequence is called hard decision, and its output is called a hard decision value.
On the other hand, when the modulated signal is demodulated, outputting the demodulated signal as a received signal with a likelihood value of a signal bit of 0 or 1 is called a soft decision output, and its output is called a soft decision value. A received signal output as a hard decision value or a soft decision value is output as a final received bit sequence by a decoder. Among decoders, a decoder can select whether an output is a hard decision value or a soft decision value as in a turbo decoder.
On the other hand, an area covered by one base station in a mobile communication system is referred to as a cell, and the coverage area thereof generally overlaps between adjacent cells. Thus, when a terminal device is located near a cell edge, a phenomenon that a radio signal transmitted from the terminal device to a desired base station and a radio signal transmitted from the terminal device belonging to an adjacent base station interfere with each other and a transmission rate between the terminal device and the base station is significantly degraded becomes a problem. Coordinated multi-point transmission/reception (CoMP) technology (see, for example, Non-Patent Document 3) has been studied as means for solving the problem of inter-cell signal interference.
FIG. 5 is a schematic diagram of CoMP using two base stations 4 and 5. FIG. 6 is a schematic diagram of CoMP using one central unit 1 and two distributed units 2-1 and 2-2.
As illustrated in FIG. 5, in CoMP, adjacent base stations 4 and 5 communicate with each other in coordination with a terminal device 6 located near the cell edge. In FIG. 6, the distributed units (base stations) 2-1 and 2-2 connected to the adjacent central unit 1 communicate with each other in coordination with the terminal device 6 located near the cell edge.